In a parts mounting process for mounting electronic parts on a substrate, the substrate is positioned and held while its downside is supported by a downside receiving section. A lower supporting pin method for supporting a downside of a substrate while a plurality of lower supporting pins remain in contact with the downside has widely been used as the method for receiving a downside of a substrate. Under the lower supporting pin method, when parts have already been mounted on a downside of a substrate in a preceding process, the lower supporting pins must be positioned by selecting locations where the lower supporting pins can receive the downside without interfering with the positions of the already-existing parts. After completion of work for positioning pins, a check is performed as to whether or not the lower supporting pins are actually positioned at the thus-selected correct locations. A method known as the method for checking the positions of the lower supporting pins is for recognizing leading ends, which remain in contact with the substrate, with a camera, thereby detecting positions of the lower supporting pins (see; for instance, Patent Document 1).